C++ random number same sequence every time

Question

I wrote this to generate a random password:

#include <iostream>
#include <fstream>
#include <string>
#include <cstdlib>
using namespace std;

string read(string value) { //read value
    ifstream input;
    int olength;
    string line = "", output = "";
    size_t pos;
    bool a = true;
    int i = 0;
    input.open("pg_options.txt");
    if (!input.is_open()) {
        cout << "pg_options.txt missing.";
        return "error";
    }
    while (getline(input, line)) {
        pos = line.find(value);
        if (pos != string::npos) {
            while (a == true) {
                if (line[i] == '=') {
                    i++;
                    break;
                }
                else {
                    i++;
                }
            }
            olength = line.length() - value.length() - 1;
            for (int i2 = 0; i2 < olength; i2++) {
                output += line[i];
                i++;
            }
        }
    }
    input.close();
    return output;
}

char randupper() { //generate random upper case character
    char uppercase[26] = { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z' };
    int i = rand() % 26;
    return uppercase[i];
}

char randlower() { //generate random lower case character
    char lowercase[26] = { 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z' };
    int i = rand() % 26;
    return lowercase[i];
}

char randspecial() { //generate random special character
    char special[7] = { '!', '#', '$', '%', '&', '*', '?' };
    int i = rand() % 7;
    return special[i];
}

char randnumbers() { //generate random number
    char numbers[10] = { '1', '2', '3', '4', '5', '6', '7', '8', '9', '0' };
    int i = rand() % 10;
    return numbers[i];
}

void generate() { //generate the password
    string output = "";
    int i1=0, digits = 0, upper = 0, lower = 0, special = 0, numbers = 0;
    digits = stoi(read("digits")); //get digits
    if (read("include_upper_case") == "true") { //determine number of upper case characters
        upper = rand() % (digits / 2) + 1;
        digits -= upper;
    }
    if (read("include_lower_case") == "true") {
        lower = rand() % (digits / 2) + 1;
        digits -= lower;
    }
    if (read("include_special_characters") == "true") {
        special = rand() % (digits / 2) + 1;
        digits -= special;
    }
    if (read("include_numbers") == "true") {
        numbers = digits;
    }
    else { //if numbers not included
        if (read("include_upper_case") == "true") {
            upper += digits;
        }
        else if (read("include_lower_case") == "true") {
            lower += digits;
        }
        else if (read("include_special_characters") == "true") {
            special += digits;
        }
        else {
            cout << "error generating, please check your options.";
            return;
        }
    }
    for (int i = 0; i < stoi(read("digits")); i++) {
        i1 = rand() % 4;
        if (i1 == 0) { //if uppercase
            if (upper > 0) {
                output += randupper();
                upper--;
            }
            else {
                i--;
            }
        }
        else if (i1 == 1) {
            if (lower > 0) {
                output += randlower();
                lower--;
            }
            else {
                i--;
            }
        }
        else if (i1 == 2) {
            if (special > 0) {
                output += randspecial();
                special--;
            }
            else {
                i--;
            }
        }
        else if (i1 == 3) {
            if (numbers > 0) {
                output += randnumbers();
                numbers--;
            }
            else {
                i--;
            }
        }
    }
    cout << output;
}

int main() {
    generate();
    return 0;
}

pg_options.txt:

include_special_characters=true
include_upper_case=true
include_lower_case=true
include_numbers=true
digits=10

However, it generates the same thing every time it runs, that is HM*nfx375g, so it has same sequence of random numbers. (in generate(), upper is always 2, lower is always 4, special is always 1, numbers is always 3...) Is there a function that can generate different random numbers every time?

Answer 1

Random generators need to be seeded to have a random starting point. In your case, it needs to be done by calling srand() before using rand(), for example:

#include <cstdlib>
#include <ctime>
using namespace std;

int main() {
    srand(time(0)); // <-- add this!
    generate();
    return 0;
}

However, the preferred C++ way of generating random numbers is to use the functions and types available in the <random> header file. srand() and rand() are basically a leftover from C.

See this example:

#include <iostream>
#include <vector>
#include <string>
#include <random>

// get 'entropy' from device that generates random numbers itself
// to seed a mersenne twister (pseudo) random generator
static std::mt19937 generator(std::random_device{}());

char random_lowercase_char()
{
    static std::uniform_int_distribution<std::size_t> distribution('a', 'z');
    return static_cast<char>(distribution(generator));
}

char random_uppercase_char()
{
    static std::uniform_int_distribution<std::size_t> distribution('A', 'Z');
    return static_cast<char>(distribution(generator));
}

char random_number_char()
{
    static std::uniform_int_distribution<std::size_t> distribution('0', '9');
    return static_cast<char>(distribution(generator));
}

char random_special_char()
{
    static std::vector<char> specials{ '!', '#', '$', '%', '&', '*', '?' };
    static std::uniform_int_distribution<std::size_t> distribution(0, specials.size()-1);
    auto index = (distribution(generator));
    return specials[index];
}

int main()
{
    for (int i = 0; i < 40; i++)
    {
        std::cout << random_lowercase_char();
        std::cout << random_uppercase_char();
        std::cout << random_number_char();
        std::cout << random_special_char();
        std::cout << std::endl;
    }
}

Answer 2

First add the <ctime> library

Then use srand(time(NULL)); before generating any random numbers with rand().